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Should I wake her?

DD (4 and a half months) had a terrible night last night with the start of what seems a bad cold. She is pretty miserable and has slept quite a bit of the day today. (now 2:30) Worried she won't sleep tonight if I let her sleep too long today. She is normally a very good sleeper. Am loathe to wake her as she obviously needs the sleep but don't want another night like last night.

Maybe start making noises in her room and see if she stirs??
I'm sure she needs it though and will still sleep the same tonight if you stick to your bedtime routine. or maybe just put her down a little later.

Wake her. Unless you're happy to accept whatever waking happens during the next night or more. I find that it's always best to try and stick to routine, the children seem far more settled that way. She can always have another brief nap later this afternoon if she needs.